1. Field of the Invention
The present invention relates to a taking lens, and more particularly, to a taking lens comprising a focus determination optical system which splits object light and allows the split light to enter a focus status determination image-capturing element.
2. Description of the Related Art
Various conventional methods are proposed for determining a focus status (front-focused, rear-focused, just-focused) of a taking lens using a plurality of image-capturing elements with different optical path lengths (e.g., Japanese Patent Application Publication No. 55-76312 and Japanese Patent Publication No. 7-60211). For example, for an image-capturing element for capturing video pictures (video image-capturing element), two focus status determination image-capturing elements in the same picture-taking range are placed in a position whose optical path length is longer and a position whose optical path length is shorter than that of the video image-capturing element, respectively. Then, according to high frequency components of the image signals obtained from these focus status determination image-capturing elements, focus estimates for the respective image-capturing planes of the respective focus status determination image-capturing elements are calculated and compared. Thus, according to a comparison between the focus estimates, the focus status of the image-capturing plane of the video image-capturing element is determined, that is, whether it is front-focused, rear-focused or just-focused status. Such a focus status determination method is applicable to focus determination for auto focusing, etc.
The object light for focus status determination can be obtained by splitting the object light for picture-taking that passes through the taking lens. In this case, in addition to a normal picture-taking optical system, a focus status determination optical system is constructed for the taking lens. The taking lens in such a configuration needs to place the image-capturing plane of the image-capturing element that picks up the object light for picture-taking and the image-capturing plane of the image-capturing element that picks up light for focus status determination in positions conjugate with each other. This is because if these image-capturing planes are not in positions conjugate with each other, autofocus would operate in such a way as to focus on the image-capturing element for focus status determination, with the result that the taken picture of the object is always out of focus, causing a problem of deteriorating focus accuracy.
For this reason, the product is conventionally shipped with the image-capturing plane of the image-capturing element for focus status determination adjusted to a position conjugate with the image-capturing plane of the image-capturing element for picture-taking and fixed at that position.
However, due to environmental variations in temperature and humidity, etc., the image-capturing plane of the image-capturing element for picture-taking may deviate from the position predetermined at the time of shipment. In that case, the focus position on the picture-taking side can be readjusted by adjusting tracking through a conventionally used mechanism that moves a master lens, etc., whereas the focus position on the focus status determination side remains deviated due to the lack of a readjustment mechanism.